Free particle abrasion development of imaging sheets employing photosensitive microcapsules

ABSTRACT

Imaging sheets having a coating containing chromogenic material and a photosensitive composition, with at least the photosensitive composition encapsulated in rupturable microcapsules as an internal phase, are processed by free particle abrasion to rupture the microcapsules coating the surfaces of the imaging sheets by establishing relative motion between the imaging sheets and a body of free particles.

This is a division of application Ser. No. 670,447, filed Nov. 13, 1984,now U.S. Pat. No. 4,578,340.

BACKGROUND OF THE INVENTION

The present invention relates generally to an imaging system utilizingimaging sheets having a surface coating of rupturable photosensitivemicrocapsules and, more particularly, to a method and apparatus forprocessing such imaging sheets.

U.S. Pat. Nos. 4,440,846 and 4,399,209, which are assigned to The MeadCorporation and herein incorporated by reference, describe an imagingsystem wherein a photosensitive layer comprising microcapsulescontaining a photosensitive composition in the internal phase isimage-wise exposed to actinic radiation and subjected to a uniformrupturing force whereupon the microcapsules rupture and image-wiserelease the internal phase. The imaging system is particularlyadvantageous because it is a totally dry system and does not rely uponthe application of wet development processing solutions to produce theimage. An image-forming chromogenic material, such as a substantiallycolorless color former, is typically associated with the microcapsules.When the microcapsules rupture, the color former image-wise reacts witha developer material and produces a color image. In the embodimentsdescribed in the referenced patents, the microcapsules are typicallyruptured by passing image-wise exposed imaging sheets through the nipbetween a pair of calender rollers.

While heavy pressure is not required to rupture the microcapsules, highpressure and large calender rollers are normally used to develop theimaging sheets. Even carefully machined metal calender rollers haveuneven surfaces. If one roller is simply rested upon another, thesurfaces of the rollers are not in contact over the entire length of therollers. By applying pressure to the rollers, the uneven surfaces orsurface irregularities are "smoothed out" to provide a uniform contactline between the rollers. The high pressure and large size of therollers are necessary to achieve a uniform distribution of the rupturingforce across the surface of the imaging sheets. If the rupturing forceis not uniformly distributed, the imaging sheets develop unevenly andthe tonal characteristics of the resulting images are not good.

As the width of the imaging sheets and the corresponding length of thecalender rollers increases, the diameter of the rollers must also beincreased to maintain sufficient stiffness such that pressure applied tothe ends of the rollers is distributed evenly across the entire expanseof the rollers. As a general rule, as the length of the rollers isdoubled, the diameter of the rollers must be cubed in order to maintainsufficient stiffness. Thus, as the size of the imaging sheets increases,the cost and size of effective pressure type development apparatusbecomes prohibitive. In particular, while larger pressure rollers may beaccommodated in development apparatuses designed to develop relativelysmall copies, e.g., 8 to 12 inches in width, in certain applicationssuch as color proofing, large copy widths, sometimes in excess of 36inches, are used. These copies are so large as to make it impractical todevelop them using pressure rollers.

An additional drawback of pressure processing imaging sheets is thatwhen high pressures are applied to the calender rollers, as required toovercome surface irregularities and achieve uniform development acrossthe roller, some of the midtone quality of the developed images is lost.This is apparently due to a loss of a differential microcapsule ruptureand is referred to as "midtone mottle". Tnus, a number of difficultieshave been encountered in designing pressure type development apparatusfor the aforementioned imaging sheets.

An improved arrangement for processing imaging sheets without highpressure and bulky calender rollers is disclosed in commonly assignedU.S. Pat. No. 4,448,516, which is herein incorporated by reference. Inthis patent, the imaging sheets are passed over a developer roll havinga fibrous outer surface. The developer roll is rotated in contact withthe imaging sheets to rupture the microcapsules on the sheets. Thedeveloper roll is an improvement over the bulky high pressure calenderrollers and has advantages over the alternative proposals disclosed inthe first referenced patents.

New alternatives which offer inexpensive techniques for developingimaging sheets and may be preferred for selected applications are indemand and serve to advance the art of imaging systems utilizing theimaging sheets of the first referenced patents.

SUMMARY OF THE INVENTION

The term "microcapsule" as used herein refers to both microcapsuleshaving a discrete microcapsule wall and microcapsules formed in aso-called open phase system the internal phase constituents are simplydispersed in a binder.

The term "photosensitive composition" means a composition which changesviscosity upon exposure to actinic radiation.

The term "chromogenic material" refers to the color forming reactantwhich is encapsulated or otherwise associated with the microcapsules.The term "developer" refers to the reactant not associated with themicrocapsules.

In accordance with the present invention, imaging sheets are processedby free particle abrasion. In its broadest sense, the present inventionrelates to a method and apparatus for rupturing the microcapsules whichform a photosensitive layer of the aforementioned imaging sheets byforming a body of free particles and establishing relative movementbetween the surface of an imaging sheet and the body of free particlessuch that the free particles move over the microcapsules on the surfaceof the imaging sheet and thereby abrade and rupture the microcapsules.The latent image in the imaging sheet is developed if the microcapsulesare ruptured in the presence of a developer. A developer can be providedas a coating on the imaging sheet itself or may be intermixed with thebody of free particles such that as the microcapsules are ruptured, thedeveloper and a chromogenic material, such as a colorless color former,interact and form the image. As a further alternative, the microcapsulescan be ruptured by contacting a first body of free particles and thedeveloper can be applied later.

In a first embodiment of the present invention, an imaging sheet isplaced into a tray with the body of free particles and the tray isvibrated such that free particles move across the surface of the imagingsheet abrading and rupturing the microcapsules thereon.

A second embodiment for performing free particle abrasion development ofimaging sheets comprises a tray for supporting a body of free particlesand a cylinder mounted for rotation above the tray and located relativeto the tray such that a sector of the circumference of the cylinderextends into the body of free particles. An imaging sheet is processedby securing it to the cylinder and rotating the cylinder such that thesurface of the imaging sheet contacts the body of free particles. Thisembodiment may be enhanced by vibrating the tray as the cylinder isrotated.

A third embodiment for performing free particle abrasion development ofimaging sheets in accordance with the present invention comprises aprimary cylindrical drum mounted for rotation about a horizontal axis.In this embodiment, an imaging sheet is positioned along the interiorsurface of the drum and the body of free particles is placed into thedrum. The drum is then rotated such that free particles move over thesurface of the imaging sheet and abrade and rupture the microcapsulesthereon. Preferably, the speed of rotation of the primary cylindricaldrum is selected such that the free particles are carried partially upthe interior surface of the drum and then return toward the bottom ofthe drum in an oscillatory wave-like motion.

To ensure proper operation of the third embodiment of the presentinvention, the imaging sheet or substrate may be retained substantiallyagainst the interior surface of the drum by means of clamping members.Alternately, a secondary cylindrical substrate retaining drum may bepositioned concentrically within the primary cylindrical drum, with thesubstrate and the free particles being positioned between the primaryand secondary drums. In each of the aforementioned embodiments, thedeveloper can be carried on the imaging sheet, be intermixed with thebody of free particles hence applied from the body of free particles orbe applied separately after the microcapsules are ruptured.

It is, therefore, an object of the present invention to provide a methodand apparatus for processing imaging sheets by means of free particleabrasion such that microcapsules on imaging sheets are contacted by freeparticles in the presence of a developer to thereby rupture themicrocapsules and permit chromogenic material associated with themicrocapsules to interact with the developer and thereby develop latentimages in the imaging sheets.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C show a self-contained imaging sheet and the processing ofthe imaging sheet in accordance with the present invention.

FIGS. 2A-2C show a transfer imaging sheet and the processing of thetransfer imaging sheet in accordance with the present invention.

FIGS. 3-6 are schematic cross-sectional views of illustrativeembodiments of apparatus for processing imaging sheets in accordancewith the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 2A illustrate embodiments of the imaging sheets inaccordance with referenced U.S. Pat. Nos. 4,440,846 and, 4,399,209,respectively. Therein, an imaging sheet 10 is constituted by a substrate12 coated with a layer of microcapsules 14. The microcapsules 14 arefilled with an internal phase 16 containing a photosensitivecomposition. Usually, the microcapsules also contain the chromogenicmaterial, however, the chromogenic material can be associated with themicrocapsules in other ways such as by incorporation into themicrocapsule wall or in a layer contiguous with the microcapsules.

In actuality, the microcapsules 14 are not visible to the unaided eyesince the mean size of the microcapsules generally ranges fromapproximately 1 to 25 microns. In the imaging sheet 10 shown in FIG. 1A,a layer of developer material 20 is interposed between the layer ofmicrocapsules 14 and the substrate 12.

Exposure of the imaging sheets 10 by transmission imaging is shown inFIGS. 1B and 2B wherein a source of radiant energy 22 is positionedabove the surface of the imaging sheets 10 with a mask 24 positionedtherebetween. In the illustrations of FIGS. 1 and 2, the substrate 12 isopaque and the photosensitive material within the microcapsules 14 is apositive working radiation-curable material, i.e., the viscosity of thematerial increases upon exposure to actinic radiation.

Irradiation of the exposed areas 26 causes the radiation curablecomposition in the internal phase 16 of the microcapsules 14 topolymerize thereby gelling, solidifying or otherwise immobilizing thechromogenic material and preventing the chromogenic material fromreacting with the developer material. To simplify the illustration,internal phase 16' in the exposed areas 26 is shown as a solid; whereas,the internal phase 16 remains liquid in the unexposed areas 28.

FIGS. 1C and 2C illustrate schematically the processing of the imagingsheets 10 in accordance with the present invention to rupture themicrocapsules 14 such that the chromogenic material contained within themicrocapsules 14 or otherwise associated therewith can interreact withdeveloper. The imaging sheets 10 are processed by free particle abrasionto rupture the microcapsules 14 by shear forces parallel to the surfaceof the imaging sheets as opposed to pressure forces normal thereto.

A body of free particles 29 shown in FIGS. 3-6 and comprising individualparticles 30 contact the coated surface of the imaging sheets 10 tothereby rupture the microcapsules 14 by means of shear forces which areapplied to the microcapsules 14 as the imaging sheet 10 and theparticles 30 of the body of free particles 29 are moved relative to oneanother. The microcapsules 14, the particles 30 and the developer 20,20Aare not shown to size either actual, as noted above, or relative, butare shown to more clearly illustrate the invention of the presentapplication. As the particles 30 move over the microcapsules 14, theyabrade and thereby rupture the microcapsules.

For simplification, the microcapsules 14 are shown as being ruptured inthe unexposed area 28 and unruptured in the exposed area 26. Inactuality, all or a portion of the microcapsules may also be ruptured inthe exposed area 26. In unexposed areas 28, the chromogenic material anda developer react to form a visible image 32.

As shown in FIG. 1C where an intermediate layer of developer 20 ispositioned between the substrate 12 and the layer of microcapsules 14,the chromogenic material from the ruptured microcapsules in theunexposed areas 28 passes to the portions of the developer 20 which formthe visible image 32 by interaction between the chromogenic material andthe developer. For the imaging sheet 10 shown in FIG. 2C which isnormally utilized for transfer imaging, finely divided particles 20A ofa developer are interspersed with the particles 30 of the body of freeparticles 29 such that when th microcapsules 14 in the unexposed areas28 of the microcapsules 14 are ruptured, the finely divided developer20A mixes and interreacts with the chromogenic material contained withinor associated with the ruptured microcapsules 14 to form the image 32.As an alternate method of applying the developer, it can be applieddownstream of the free particle body by dusting or by application from aseparate sump.

The size and density of the particles 30 must be maintained withinlimits. In particular, if the particles become too large, the number ofpoint contacts with the imaging sheet is reduced and it becomes moredifficult to develop the image or the development time becomesexcessive. Similarly, if the particles are too small, they do not havesufficient weight and consequently, there is not enough shear force torupture the microcapsules and develop the imaging sheet. More denseparticles are preferred. The presently preferred material for making theparticles is iron although various sands have also been successfullyused. The recommended particle size ranges from about 250 to 750 micronsand the recommended density from about 2.0 to 5.0.

The shape of the particles 30 is also a consideration in the presentinvention. Surprisingly, preliminary results indicate that the particles30 are preferably round as opposed to irregular or angularly shaped. Itis also desirable to coat the particles with various polymers. In thecase of iron particles, the coating prevents rust. Such coatings alsofunction to increase friction and to repel oil that is released from themicrocapsules upon rupture. For the noted microcapsule size range of1-25 microns, preliminary results indicate that nearly roundapproximately 1 mm in diameter iron balls coated with a vinyl polymerare preferred.

Even if the particles 30 are coated with a polymer, oil still tends toaccumulate on the particles. After a given amount of oil accumulation onthe particles, background coloring (fog) begins to increase.Accordingly, after a number of sheets have been developed in a givenbody of free particles, the free particles must be replaced similar tothe addition of toner in a typical xerographic reproduction machine. Infact, if the transfer imaging sheet shown in FIGS. 2A-2C is used, as thefree particles are changed, the developer 20A is replenished againresembling the replacement of toner in xerographic reproductionmachines.

A variety of illustrative embodiments of apparatus for performing freeparticle abrasion to rupture microcapsules coating the surfaces ofimaging sheets are shown in FIGS. 3-6. In the simplest form of apparatusfor performing free particle abrasion, a substrate or imaging sheet 40is placed into a tray 42, with the body of free particles 29 also placedinto the tray 42 over the imaging sheet 40. The tray 42 is then vibratedas indicated by an arrow 43 such that the free particles 30 of the bodyof free particles 29 move over the surface of the imaging sheet 40 tothereby abrade and rupture the microcapsules forming a layer on theupper surface of the imaging sheet 40. The tray 42 is vibrated with avibration rate of approximately 100 hertz, the presently preferred rateof vibration.

A second illustrative embodiment of apparatus for performing freeparticle abrasion of imaging sheets is shown in FIG. 4. Therein, a tray44 supports the body of free particles 29 and a cylinder 46 ishorizontally mounted above the tray 44 for rotation about an axis 47.The cylinder 46 is located relative to the tray 44 such that a sector 48of the circumference of the cylinder 46 extends into the body of freeparticles 29. An imaging sheet 50 is secured to the cylinder 46 suchthat the surface of the imaging sheet 50 which is mounted with themicrocapsules facing outwardly. The imaging sheet 50 is then processedby rotating the cylinder 46 such that the coated surface of the imagingsheet 50 contacts the body of free particles 29. The operation of theembodiment of FIG. 4 may be enhanced by vibrating the tray 44 as thecylinder 46 is rotated, such vibration being indicated by an arrow 52.

A third embodiment of apparatus for performing free particle abrasion ofimaging sheets in accordance with the present invention is shown in FIG.5. In this embodiment, a primary cylindrical drum 54 is mounted forrotation about a horizontal axis 56. An imaging sheet 58 is positionedalong the interior of the surface of the drum 54 by means of insertionthrough a slot 60 which is angled in the direction of drum rotationindicated by the arrow 62. The imaging sheet 58 can be held against theinside of the drum 54 by means of clamping members 64 which are optionaland generally not necessary.

As the drum 54 is rotated, the free particles 30 of the body of freeparticles 29 move over the surface of the imaging sheet 58 to abrade andrupture the microcapsules thereon. Preferably, the speed of rotation ofthe primary cylindrical drum 54 is limited such that the particles 29 donot tumble, i.e., the particles are not thrown around in the drum, andare not held against the interior of the drum 54 by centrifugal forces.Rather, the speed is selected such that the free particles are carriedpartially up the interior surface of the drum and then return toward thebottom of the drum in an oscillatory wave-like motion defining aswishing action.

A fourth embodiment of apparatus for performing free particle abrasionof imaging sheets in accordance with the present invention is shown inFIG. 6. This embodiment comprises a modification of the embodiment ofFIG. 5 and, accordingly, corresponding elements will be labeled the samein FIGS. 5 and 6. In FIG. 6, a secondary cylindrical substrate retainingdrum 66 is positioned concentrically within the primary cylindrical drum54, with the imaging sheet 58 and the body of free particles 29 beinginserted between the primary drum 54 and the secondary drum 66. In theembodiment of FIG. 6, the imaging sheet 58 is retained substantiallyagainst the interior surface of the primary drum 54 by means of thesecondary drum 66.

The drawings of FIGS. 3-6 are drawn to schmatically illustrate apparatusin accordance with the present invention and are not drawn to scalewhich, to a large extent, is determined by the imaging sheets to beprocessed. In particular, the dimensions of the drum and the drumarrangement of FIG. 6 are such that the interior or secondary drum 66 isapproximately 1 inch smaller in diameter than the outer or primary drum54. Further, it should be noted that there is no reason that an imagingsheet cannot extend completely around and entirely cover the interiorsurface of the primary drum 54 since the developing action performed inaccordance with the present invention is uniform throughout the interiorsurface of the primary drum 54.

It is apparent that a method and inexpensive apparatus for processingimaging sheets by means of free particle abrasion such thatmicrocapsules on the imaging sheets are contacted by free particles of abody of free particles in the presence of a developer to thereby rupturethe microcapsules and permit chromogenic material coating the sheets tointeract with the developer and thereby develop latent images on theimaging sheets has been disclosed. While a variety of embodiments havebeen disclosed for performing free particle abrasion of imaging sheets,many other arrangements will be apparent to those skilled in the artafter a review of the above disclosure.

While the methods herein described and the forms of apparatus forcarrying these methods into effect constitute preferred embodiments ofthis invention, it is to be understood that the invention is not limitedto these precise methods and forms of apparatus and that changes may bemade in either without departing from the scope of the invention whichis defined in the appended claims.

What is claimed is:
 1. Apparatus for rupturing photosensitivemicrocapsules which form a layer on the surface of an imaging sheetcomprising:a body of free particles; and mobile means receiving saidimaging sheet for establishing relative movement between said imagingsheet and said free particles such that said free particles move oversaid imaging sheet and rupture said microcapsules.
 2. Apparatus forrupturing microcapsules as claimed in claim 1 wherein said mobile meanscomprises a tray mounted for vibratory motion and adapted to receivesaid imaging sheet and said free particles.
 3. Apparatus for rupturingmicrocapsules as claimed in claim 1 further comprising a tray forreceiving said free particles and wherein said mobile means comprises acylinder mounted for rotation above said tray, said cylinder beingadapted to receive said imaging sheet and positioned relative to saidtray such that said imaging sheet passes through said free particles assaid cylinder is rotated.
 4. Apparatus for rupturing microcapsules asclaimed in claim 3 wherein said tray is mounted for vibratory motion. 5.Apparatus for rupturing microcapsules as claimed in claim 1 wherein saidmobile means comprises a primary hollow cylindrical drum mounted forrotation and adapted to receive said imaging sheet and said freeparticles therewithin such that said free particles move over saidmicrocapsules and rupture them as said primary cylinder is rotated. 6.Apparatus for rupturing microcapsules as claimed in claim 5 wherein saiddrum comprises clamp means for securing said imaging sheet against theinterior thereof.
 7. Apparatus for rupturing microcapsules as claimed inclaim 5 further comprising a secondary cylindrical drum concentricallypositioned within and spaced apart from said primary drum, said freeparticles and said imaging sheet being received between said primary andsecondary drums such that said microcapsules are engaged by said freeparticles as said primary drum is rotated and said imaging sheet isretained substantially adjacent the interior wall of said primary drumby said secondary drum.